In 1929 Edwin Hubble measured the red shifts and distances of many galaxies. He found that almost all galaxies are receding, and that the further away a galaxy is, the faster it is moving away. Hubble's law states:v = Hd where v is the recessional velocity of the galaxy, d is its distance and H is the Hubble constant. The velocity comes from the red shift (v = zc) and the distance from standard candles such as type Ia supernovae.A graph of v against d is a straight line through the origin with gradient equal to H.
The value used in AQA calculations is:H = 65 km s-1 Mpc-1 In SI units: H = 65 × 103 / 3.08 × 1022 = 2.1 × 10-18 s-1 (1 Mpc = 3.08 × 1022 m).Hubble's law shows that the universe is expanding. It is not that we are at the centre: from any galaxy, the others would appear to recede in the same way, as space itself is stretching.Worked example: a galaxy is 200 Mpc away.
v = 65 × 200 = 13 000 km s-1
Its red shift is z = v/c = 1.3 × 107 / 3.00 × 108 = 0.043Hubble's law can also be used the other way: measuring the red shift of a very distant galaxy gives its velocity, and d = v/H then gives its distance.
The value used in AQA calculations is:v = 65 × 200 = 13 000 km s-1
Its red shift is z = v/c = 1.3 × 107 / 3.00 × 108 = 0.043Hubble's law can also be used the other way: measuring the red shift of a very distant galaxy gives its velocity, and d = v/H then gives its distance.